Wednesday, November 5, 2014

Britt Fiscus

Dugway Geodes

On October tenth and eleventh I went on a field trip with the Snow CollegeGeology FieldStudies class. One of the stops we made on the tenth was to the Dugway Geode Beds in Juab County. We drove a couple of miles into the beds to find a promising pit, then started searching and digging for geodes. It was a very successful trip and we all took home several geodes. Finding these beautiful crystal-filled rocks made me curious about how they form.

I found out that they form in two different ways. They can form in sedimentary rocks when organic matter such as a tree root, rots away and leaves a cavity in the ground. If the cavity is still preserved in the ground after the sediment becomes rock, it has the potential to become a geode. (Baggaley, 2012) To become a geode, water has to find its way into the cavity of the rock to deposit the minerals that are necessary to create crystals. After millions of years of mineral rich ground water running through tiny cracks in the rock, crystals will form in the cavity.

The other way that geodes can form is in volcanic rock. Cooling lava or magma often contains gases that are trapped in bubbles. After the lava solidifiesinto a rock the process is the same as sedimentary geodes. The geodes formed from water – in this case water that was hydrothermal (heated by the magma in the area) and so it contained a lot of dissolved silica. This silica rich water seeped into holes left by the gases and precipitated quartz. This is the most common type of crystal found in geodes. They were formed in volcanic rock called rhyolite. According to http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm they were formed in rhyolite that formed 6 to 8 million years ago. Then, around 32,000 to 14,000 years ago Lake Bonneville covered western Utah. The lake’s activity eroded the rhyolite that the geodes are found in and redeposited it as lake sediment where we find them today.(Ege, UGS.gov)

Starting October 10th and ending on the
11th the GEO 2500 class went on a two day geology marathon full of traveling on
rough dirt roads, hiking, rock hunting and finding. We also learned about some
of the major geological events that created the variety of terrain we
encountered. We left Ephraim at approximately 8:45 am on the 10th and headed
westward to Topaz Mountain. Roughly 116 miles later we arrived at Topaz
Mountain. Almost immediately we put on our safety goggles, grabbed hammers and
chisels and got to work looking for topaz. We had a variety of success finding
topaz, and while looking closely at some grayish white rhyolite I stumbled upon
a small black crystal called pseudobrookite.

Pseudobrookite, which is Greek for false
brookite, is a rare oxide mineral which usually forms by pneumatolytic
processes or by reactions with xenoliths in titanium-rich andesite, rhyolite,
basalt, according to rruff.info/doclib/hom/pseudobrookite.pdf.
According to dictionary.com and merriam-webster.com,
pneumatolytic means formed or forming by hot vapors or super heated liquids
under pressure, the process by which rocks are altered or minerals and ores are
formed by the action of vapors given off by magma. Pseudobrookite
is often found with hematite, magnetite, bixbyite, ilmenite,
enstatite-ferrosilite, tridymite, quartz, sanidine and topaz. The topaz at
Topaz Mountain formed in vugs located in a lava flow, and the pseudobrookite
formed in the same rhyolite as the topaz in a similar process. Pseudobrookite , which has the chemical formula
Fe2TiO5, often has either a brownish-black, a reddish
brown, or a black color. It is opaque with a metallic luster (webmineral.com).According to http://www.mineralmarket.com/TopazMtn/topaz7.html

Pseudobrookite crystals are skinny elongate prisms with striations (look like
grooves) and belong to the orthorhombic crystal system.

The pseudobrookite was an unexpected find
since we were looking for quartz and topaz. We got home late that night tired
but successful in finding multiple crystals, geodes, and trilobites. The next
day we left later in the day and headed to eastern Utah. We managed to find heaps
of gypsum after a long day of traveling, which was very exciting. The two day
road trip was a very enjoyable adventure.